Long-haul optical communication systems e.g. systems of lengths greater than about 600 kilometers, suffer from signal attenuation resulting from a variety of factors, including scattering, absorption, and bending. To compensate for attenuation, long-haul systems may include a series of optical amplifiers spaced along the transmission path between a transmitter and a receiver. The amplifiers amplify the optical signal in a manner allowing reliable detection at the receiver.
Erbium doped fiber amplifiers (EDFAs) have proven particularly useful in long-haul systems. In general, an EDFA includes an erbium-doped fiber segment that is “pumped” with light from one or more pump sources. The pump source, e.g. a laser, excites erbium atoms in the doped segment, which then serve to amplify the optical signal passing therethrough.
Raman amplifiers are also known. Raman amplification occurs throughout an optical transmission fiber segment when it is pumped at an appropriate wavelength or wavelengths. Each Raman amplifier may contain one or more pumps. Gain is achieved over a spectrum of wavelengths longer than the pump wavelength through the process of Stimulated Raman Scattering. Although the power efficiency associated with Raman amplifiers is less than that achieved by EDFAs, Raman amplifiers may provide better noise performance by means of distributed amplification in the transmission fiber.
Hybrid Raman/EDFA (HRE) amplifiers combine the features of both Raman and EDFA amplifiers. In a HRE a Raman portion typically acts as a pre-amplifier before an EDFA. The addition of Raman amplification to an EDFA configuration allows for an economical increase in spacing between amplifiers on the transmission path compared to use of EDFAs alone.
In a wavelength division multiplexed (WDM) optical communication system several optical signals are transmitted on the same fiber at different wavelengths/channels. Ideally, the optical amplifiers in a WDM system should amplify each channel within the system bandwidth at the same level of gain. If the transmitted channels are non-uniformly amplified, channels may ultimately be lost due to progressive under-amplification as they propagate in the transmission path.
Unfortunately, EDFAs and Raman amplifiers, and consequently HREs, exhibit non-flat gain characteristics across the system bandwidth associated with WDM systems. Gain flattening or gain equalization may, however, be achieved by the use of gain flattening filters. Gain flattening filters render the gain level substantially flat across the system bandwidth. In a multi-stage EDFA, for example, a gain flattening filter may be provided between EDFA stages. In HRE configurations, where the EDFA is often of a single-stage design, gain flattening filters have been provided at the output of the EDFA portion of the HRE.
Long-haul undersea systems may consist of very long spans and subsequently be pump power limited. That is, the pump power available for an amplifier may be practically limited to a maximum level below that which would be necessary to achieve optimum transmission performance. In such a system incorporating HREs, in order to receive the full benefits of Raman pre-amplification, it may be desirable to make efficient use of the limited pump power.
Providing gain flattening filters at the output of the EDFA portion of an HRE (referred to herein as post-filtering), however, results in filtering of a significant portion of the power imparted by the EDFA section, effectively wasting pump power. FIG. 6, for example, includes plots 600 of signal power vs. wavelength illustrating the power lost in equalization using a post-filtering approach. Plot 602 illustrates an EDFA output power spectrum provided at the input of a gain flattening filter, and plot 604 illustrates the power spectrum at the output of the gain flattening filter. The region 606 between plot 602 and plot 604 represents the loss in signal power resulting from post-filtering. As shown, although the power spectrum 604 at the output filter is flattened compared to the power spectrum 602 at the input of the filter, gain flattening is achieved with a significant loss of signal power. In the illustrated exemplary embodiment, 5.65 dBm of signal power is effectively wasted by a post-filtering approach.
There is therefore a need for a system and method for gain equalization in a hybrid Raman/EDFA used in long-haul undersea WDM optical communication systems that allows for more efficient use of amplifier pump power. There is also a need for an undersea WDM optical communication system incorporating such a system.